This application claims the benefit of priority under 35 USC xc2xa7119 of European Patent Application Serial No. EP00124835.0, filed on Nov. 14, 2000.
The present invention relates to a temperature-stabilized optical amplifier for optical telecommunications and to a method for temperature-stabilizing an optical amplifier. In particular, the present invention relates to a temperature-stabilized fiber-optic amplifier suitable for metropolitan optical networks, i.e. optical networks typically adapted to transmit signals at distances of some kilometers or some tenths of kilometers (up to about 300 km).
As known, fiber-optic telecommunication systems are usually equipped with fiber-optic amplifier for the amplification of optical signals. A fiber-optic amplifier typically comprises an active optical fiber that is opportunely doped with ions of a rare-earth element such that signal light with wavelength of approximately 1530-1565 nm can be amplifier in the fiber if the population of the excited states of the erbium ions is such that rate of stimulated emission exceeds that of spontaneous emissions and absorption. In such a circumstance, light within the gain bandwidth entering the optical fiber will experience net gain, and will exit the fiber with greater power.
An optical pump source, coupled to the wave-guiding portion of the optical fiber, provides a pump radiation to the active fiber in order to excite the dopant ions onto an excited state. In the case of erbium doping, a high-power diode laser with emission to approximately 980 nm is typically used. Pumping at this wavelength results in low noise figure amplifiers. The relatively small cross-sectional area of the wave-guiding portion of the optical fiber helps to ensure high intensity pumping and therefore allow appreciable gain of the signal wavelengths. However, the properties of the signal produced by such an amplifier will depend to a large extent on the properties of the diode laser used to pump the fiber.
practical system, the diode lasers are permanently and robustly connected by an opto-mechanical apparatus to a length of undoped optical fiber, which in turn is connected to the doped fiber in the optical amplifier. The assembly consisting of the diode laser, the opto-mechanical apparatus and the undoped optical fiber is called a pigtailed diode laser. Presently, many pigtailed diode lasers have undesirable characteristics such as wavelength and intensity instabilities that create noise in the pumped system. One of the most troublesome sources of diode laser noise in 980 nm diode lasers is wavelength fluctuation caused by changes in temperature. This dependency is substantially linear, and a typical value of the ratio between the emission wavelength variation and temperature is about 0.33 nm/xc2x0 C. These wavelength emission variations make erbium amplifiers unstable within the usual working temperature range, i.e. approximately between 0 and 65xc2x0 C.
It is known that the gain G of an erbium-doped amplifier is a function of the absorbed pump power, which depends on the pump absorption coefficient xcex1(xcex) of the active fiber at the pump wavelength xcex. In turn, the pump absorption coefficient xcex1(xcex) can be expressed as:
xcex1(xcex)=10xc2x7log e"sgr"(xcex) Ntxcex93L 
where "sgr"(xcex) is the absorption cross-section, Nt is the erbium ion concentration, xcex93 is the confinement factor (fraction of the optical power contained in the core), and L is the length of the active fiber.
As the pump wavelength xcex varies, the pump absorption coefficient xcex1(xcex) and, therefore, the absorbed pump power, vary in accordance with the new value of the pump absorption coefficient at that new wavelength. FIG. 1 shows the typical curve of pump absorption coefficient vs. emission wavelength and the shift of the working points A and B of two different laser diodes into new working points Axe2x80x2 and Bxe2x80x2 as the temperature increases. When the temperature varies, the absorbed power fraction of the active fibers varies in accordance to the position of the working point of the laser diode on the depicted curve. FIG. 2 illustrates the linear dependency (with a slope of approximately 0.33 nm/xc2x0 C.) of the emission wavelength on temperature for a typical laser diode.
The Applicant has verified that an optical amplifier comprising an active fiber having a peak absorption of 5 dB and pumped by a single laser pump that is not stabilized can experience a pump absorption variation of more than 3.5 dB when the temperature varies in the temperature range 0-65xc2x0 C. Applicant has also noticed that laser diodes of a same type, even when obtained from the same manufacturing process, usually have slightly different emission wavelengths due to manufacturing tolerance.
In order to improve the temperature-insensitivity of the amplifier, the working temperature of the laser diode can be stabilized by using a Peltier cell, as suggested for example in U.S. Pat. No. 4,571,728, and the emission wavelength of the laser diode can be stabilized by using a fiber Bragg grating, as suggested for example in U.S. Pat. No. 5,485,481.
Applicant observes that these techniques add cost and complexity to the amplifier while, especially in the case of metropolitan optical networks, it is important to reduce the cost of the system to a minimum.
U.S. Pat. No. 5,287,216 relates to an erbium-doped fiber optic amplifier in which the doped fiber is simultaneously pumped by multiple pump lasers generating optical waves of differing wavelengths. The optical waves are combined using a wavelength division multiplexer (WDM) before introduction into the doped fiber. The WDM used for the experimental measures can combine any pump between 960 and 975 nm with another in the 985 and 1000 nm range.
The Applicant observes that, due to the use of WDM for coupling the different wavelengths, the pump arrangement of U.S. Pat. No. 5,287,216 has relatively limited tolerance intervals for the pump wavelength, and requires to duplicate the number of pump sources if an active fiber has to be pumped bidirectionally or if two different active fibers have to be pumped.
The Applicant has faced the problem of providing an optical amplifier for an optical transmission system, in particular a fiber-optic amplifier suitable for metropolitan networks, that is stable with respect to temperature, cheap and simple, and that has an efficient and versatile pumping system. In particular, the Applicant has considered the problem of temperature-stabilizing an optical amplifier without carrying out a stabilization of the pump sources, which is usually an expensive operation.
The Applicant has found that an efficient way to temperature-stabilizing, in a predetermined temperature range, an optical amplifier comprising a set of active waveguides having a pump absorption coefficient showing a maximum at a predetermined wavelength, is to pump the set of active waveguides by a mixed pump radiation comprising at least a first and a second pump wavelength lower and, respectively, higher than said predetermined wavelength at an intermediate temperature of said temperature range. In this way, as the temperature varies, the amplifier""s pump absorption coefficients at the different pump wavelengths vary with opposite signs, and the total gain of the amplifier remains substantially unchanged. The mixed pump radiation can be generated by mixing a plurality of pump beams by means of a set of optical couplers, which also split the mixed pump radiation into a plurality of power fractions used to pump at least one active waveguide bidirectionally, or at least two active waveguides monodirectionally.
This pumping scheme allows achieving a relatively high temperature-stability of the amplifier emission power, without requiring any technique for the temperature stabilization and/or the wavelength stabilization of the pump sources, and without the need of selecting a restricted subset of the manufactured pump laser diodes. Moreover, the proposed pumping arrangement is particularly versatile and reduces to a minimum the number of pump sources required to pump a predetermined number of active waveguides.
The technique of the present invention is advantageously applicable to erbium-doped fiber optical amplifiers, since for an erbium-doped fiber the relation between the pump absorption coefficient and the pump wavelength is represented by a substantially triangular curve (having a maximum at approximately 978 nm).
According to a first aspect, the present invention relates to an optical amplifier temperature-stabilized in a predetermined temperature range, comprising:
a set of active waveguides comprising at least one active waveguide, each waveguide of said set having a pump absorption coefficient showing a maximum at a predetermined wavelength;
a set of pump sources comprising at least two pump sources, for generating a plurality of pump beams at at least a first and a second pump wavelength, said first and second pump wavelength being lower and, respectively, higher than said predetermined wavelength at an intermediate temperature of said temperature range; and
a set of optical couplers comprising at least one optical coupler, optically coupling the set of pump sources to the set of active waveguides, for feeding to the set of active waveguides a mixed pump radiation comprising a fraction of each of said pump beams; the at least one optical coupler having at least two outputs optically coupled to at least one active waveguide of said set of active waveguides for feeding to the at least one active waveguide a respective power fraction of the mixed pump radiation.
The at least one optical coupler is preferably a wavelength-independent optical coupler.
The set of active waveguides preferably comprises at least one active fiber.
The set of active waveguides preferably comprises at least one erbium-doped active waveguide.
The at least two outputs of the at least one optical coupler can be optically coupled to at least two different active waveguides of the set of active waveguides. Alternatively, the at least two outputs of the at least one optical coupler can be optically coupled to opposite ends of a same active waveguide of the set of active waveguides, for feeding the respective power fractions of the mixed pump radiation to said same active waveguide bidirectionally.
The set of optical couplers can comprise a first and a second subset of optical couplers, each optical coupler of the first subset having a plurality of inputs optically coupled to respective pump sources of the set of pump sources; and each optical coupler of the second subset having a plurality of inputs optically coupled to respective outputs of respective optical couplers of the first subset, and a plurality of outputs optically coupled to at least one active waveguide of the set of active waveguides.
The at least one optical coupler can be a 3 dB optical coupler.
The wavelength distance between the first and the second pump wavelength is preferably comprised between 10 and 30 nm, more preferably comprised between 10 and 20 nm.
According to a second aspect, the present invention relates to an optical telecommunication system, comprising an optical transmitter, an optical receiver, and a fiber optic line optically connecting the optical transmitter to the optical receiver and comprising a temperature-stabilized optical amplifier as previously defined.
According to a further aspect, the present invention relates to a method for temperature-stabilized amplification of optical signals in a predetermined temperature range, comprising:
generating a first and a second pump beams having a first and a second wavelength;
mixing the first and the second pump beams for obtaining a mixed pump radiation;
splitting the mixed pump radiation into two power fractions;
feeding each of the two power fractions to an active waveguide having a pump absorption coefficient showing a maximum at a predetermined wavelength; the first and the second wavelength being lower and, respectively, higher than the predetermined wavelength in the predetermined temperature range; and
further feeding to the active waveguide the optical signals.
The step of feeding each of the two power fractions to an active waveguide can comprise feeding the two power fractions to two different active waveguides, or to a same active waveguide bidirectionally
The active waveguide is preferably an optical fiber.
The active waveguide is preferably doped with erbium.
The wavelength distance between the first and the second pump wavelength is preferably comprised between 10 and 30 nm.
Advantageously, the first and the second pump wavelengths can have substantially the same wavelength distance from the predetermined wavelength at a temperature in the middle of the predetermined temperature range.